Lisa Dillon joins Stephen Mangan for Penhall's Birthday at Court
The pair will play a couple in the tense, final hours before the arrival of their new baby.
Lisa Dillon's recent credits include The Taming of the Shrew at the RSC and Knot of the Heart at the Almeida. She is well known to Cranford fans as Miss Mary Smith in the BBC TV series.
Joe Penhall's plays include Haunted Child and Dumb Show at the Royal Court and Blue/Orange at the National Theatre which transferred to the West End and for which he received an Olivier Award.